FIG. 1 shows a typical lighting system.
A lighting system comprises a power-supply circuit 10 and at least one lighting module 20 comprising one or more light sources, such as at least one LED (Light-Emitting Diode) or other solid-state lighting means, such as laser diodes.
For example, the power-supply circuit 10 may comprise a current generator 12; namely, the power-supply circuit 10 receives at input via two terminals 100a and 100b a power-supply signal Vin (for example, from the power mains supply or a battery) and supplies a regulated current iout via a positive terminal 102a and a negative terminal 102b. 
In a complementary way, the lighting module 20 comprises a positive input terminal 200a and a negative input terminal 200b for connection to the terminals 102a and 102b of the power-supply circuit 10. For example, the lighting module 20 may be connected, directly or through a cable, to the power-supply circuit 10. Consequently, the terminal 200a is connected to the terminal 102a, and the terminal 200b is connected to the terminal 102b, and the lighting module 20 hence receives the current iout.
As shown in FIG. 2, the lighting module 20 may be a solid-state-light (SSL) module comprising a string of solid-state light sources 22 connected (for instance, directly) in series between the terminals 200a and 200b. For example, FIG. 2 shows three LEDs L1, L2, L3, which are directly connected in series between the terminals 200a and 200b. Consequently, in the example considered, the current iLED that flows through the light sources 22 corresponds to the current iout.
Frequently, the current generator 12 is provided with a linear regulator or an electronic switching converter.
For example, FIG. 3 shows a power-supply circuit 10, in which the current generator 12 is implemented with an electronic converter of the buck type, i.e., a step-down converter.
In general, an electronic converter comprises at least:                two terminals for receiving a DC voltage and two terminals for supplying a current;        a switching stage 104 comprising one or more electronic switches and one or more inductors (and possibly one or more capacitors);        a current sensor 106 configured to detect a feedback signal FB representing the current supplied by the switching stage; and        a regulation circuit 108 configured to generate one or more drive signals for the switching stage 104 as a function of the feedback signal FB.        
As explained previously, the power-supply circuit 10 receives at input a voltage Vin. In the case where this voltage is a DC voltage, for example supplied by a battery, the switching stage 104 can be directly connected to the input terminals 100a and 100b of the electronic converter. Instead, in the case where the voltage Vin is an AC voltage, for example supplied by the power mains supply, the switching stage 104 can be connected to the input terminals 100a and 100b of the electronic converter through a rectifier circuit, comprising, for example, a diode bridge and possibly a filtering circuit. Moreover, in general, an electronic converter with power-factor correction (PFC) may be connected between the switching stage 104 and the input terminals 100a and 100b. For example, in FIG. 3, the current generator, in particular the switching stage 104, receives the voltage Vin directly.
Likewise, the current supplied by the switching stage may correspond directly to the output current iout; i.e., the two output terminals of the switching stage can be directly connected to the terminals 102a and 102b of the power-supply circuit. However, also in this case, there may be provided additional filters for stabilizing the current iout. For example, in FIG. 3, the current generator, in particular the output of the switching stage 104, is directly connected to the terminals 102a and 102b. 
In the simplest case, the switching stage 104 of a buck converter comprises an electronic switch SW1, such as a field-effect transistor (FET), and an inductor L, which are connected (for example, directly) in series between the terminals 100a and 102a (positive terminals). Instead, the terminal 102b is connected to the terminal 100a (negative terminals). The buck converter further comprises a diode D connected between the terminal 100b and the intermediate point between the electronic switch SW1 and the inductor L. Consequently, in the example considered, the current iL flowing through the inductor L represents the current supplied by the current generator 12, and hence the current iout supplied through the terminals 102a and 102b. 
In the example considered, the regulation circuit 108 hence generates a drive signal DRV1 for the switch SW1.
As shown in FIGS. 4a and 4b, when the switch SW1 is closed (for example, when the drive signal DRV1 is high), the diode D is reversely biased, and the current iL flowing through the inductor L increases substantially in a linear way. Instead, when the switch SW1 is open (for example, when the drive signal DRV1 is low), the diode D is directly biased and the current iL flowing through the inductor L decreases in a substantially linear way.
To implement a current generator 12, it would hence be necessary to control the current iL supplied by the current generator 12. In particular, this can be obtained by controlling the closing or ON time TON and the opening or OFF time TOFF1 of the switch SW1.
For this purpose, the regulation circuit 108 hence detects, by means of the sensor 106, the current iL supplied by the switching stage 104. For example, FIG. 3 shows a shunt resistor RS, which is connected in series to the output terminals 102a and 102b, for example, between the terminal 102b and the anode of the diode D (i.e., the terminal 100b). Consequently, the resistor RS represents a current sensor 104 configured to detect the current iL. In general, it is possible to use also other current sensors that are able to detect signals representing the current iL, for example two current sensors configured to detect the current that flows through the switch SW1 and the diode D, respectively.
Consequently, the signal or signals FB representing the current supplied by the switching stage iL (i.e., the current supplied by the current generator 12) can be supplied to the regulation circuit 108, which drives the switch SW1 with the drive signal DRV1.
For example, solutions are known in which the drive signal DRV1 for the switch SW1 is a signal with pulse-width modulation (PWM), i.e., a signal in which the times TON and TOFF1 are variable but the duration of the switching interval TSW1=TON+TOFF1 is constant. As shown in FIG. 4b, in this case, the regulation circuit 108 may be a PI (Proportional-Integral) regulator or a PID (Proportional-Integral-Derivative) regulator, which increases or decreases the duty cycle (TON/TSW1) of the PWM signal until the mean value of the current iL corresponds to a required/reference value REF.
As shown in FIG. 4c, also known are solutions in which the regulation circuit 108 comprises a comparison circuit and in which the regulation circuit 108 closes the switch SW1 (by setting the drive signal DRV1 at a first logic level, for example high) when the current iL drops below a lower threshold THL and opens the switch SW1 (by setting the drive signal DRV1 at a second logic level, for example low) when the current iL exceeds an upper threshold THH. A comparison circuit of this sort could be implemented also with a single comparator with hysteresis.
In either case, switching of the switch SW1 typically occurs at a frequency of between 20 kHz and 200 kHz. However, whereas the switching frequency is constant for the solution described with reference to FIG. 4b, the switching frequency of the solution described with reference to FIG. 4c is variable, as a function, for example, of the load voltage.
Operation of such an electronic buck converter is well known, which renders a detailed description superfluous herein. Also known are many other electronic switching converters with other topologies (such as boost, buck-boost, flyback, forward, asymmetrical half-bridge converters, etc.), which are able to implement a current generator 12 configured to supply a regulated current iL.
As shown in FIG. 5, in many applications it may be required that the brightness of the light emitted by the lighting module 20 is adjustable (to implement the so-called dimming function) as a function of one or more dimming signals DIMM, i.e., signals indicating the required brightness. For example, the dimming signal DIMM can be received via a terminal 100c of the power-supply circuit 10 and/or be supplied by means of a sensor 112 (such as a light sensor), a user interface (for example, a trimmer) for direct variation of the dimming signal DIMM and/or a receiver (such as an infrared or radio-frequency receiver for receiving a signal transmitted by a remote control).
Also known are solutions in which the current generator 12 is configured for regulating the amplitude of the current iL; i.e., the current iL is modulated with an amplitude modulation (AM), as a function of a dimming signal DIMM. For example, in a buck converter, the regulation circuit 108 could implement such an amplitude modulation by:                varying the reference value REF of the PI/PID regulator; or        varying the thresholds THH and THL of the comparison circuit.        
Instead, FIG. 5 shows a further technique, in which the power-supply circuit 10 comprises an electronic shunt switch SW2, such as a FET, connected (for example, directly) between the terminals 102a and 102b of the power-supply circuit. The switch SW2 could also be connected (for example, directly) between the terminals 200a and 200b within the lighting module 20. In fact, in general, the switch SW2 is configured for diverting the current iL generated by the current generator 12 towards the light sources 22 or directly towards the terminal 102b, hence shorting the current generator, i.e., the output of the switching stage 104.
In this case, a control circuit 110 can hence receive at input the dimming signal DIMM and generate a drive signal DRV2 for the switch SW2 as a function of the dimming signal DIMM. In general, the control circuit 110 and the regulation circuit 108 can be implemented also in a single control circuit.
For example, as shown in FIG. 6a, frequently the drive signal DRV2 is a PWM signal, i.e., a signal in which the ON time TON2 and OFF time TOFF2 are variable but the duration of the switching interval TSW2=TON2+TOFF2 is constant. Consequently, the control unit 110 can vary the duty cycle (TON2/TSW2) as a function of the dimming signal DIMM. Consequently, as shown in FIGS. 6b and 6c, the light sources 22 of the lighting module 20 are switched on (i.e., the current iL supplied by the current generator flows through the light sources 22, iLED=iL) or switched off (i.e., the current iL supplied by the current generator does not flow through the light sources 22, iLED==0) with a pulse-width modulation. In the case where the current generator 12 is implemented with an electronic switching converter, switching of the switch SW2 typically occurs with a frequency that is low with respect to the switching frequency of the switching stage of the electronic converter 12. For example, typically, the frequency of the signal DRV2 is between 200 Hz and 2 kHz.
In general, it is also possible to combine the use of amplitude modulation and pulse-width modulation, as described, for example, in the document US 2013/0038234 A1, the contents of which are incorporated herein for reference.